Electron acceleration due to high frequency instabilities at supernova remnant shocks

TL;DR

This study uses PIC simulations to explore how electrons gain initial energy in supernova remnant shocks, revealing new acceleration mechanisms that could explain the origin of relativistic electrons observed in these environments.

Contribution

The paper demonstrates, through PIC simulations, that electrons can be pre-accelerated via Buneman instability and stochastic interactions at SNR shocks, providing a potential solution to the electron pre-acceleration problem.

Findings

Electrons are accelerated, not just heated, by Buneman instability.

Electrons can reach speeds exceeding shock-reflected ions.

Strong perpendicular electron acceleration occurs via wave-particle interactions.

Abstract

Observations of synchrotron radiation across a wide range of wavelengths provide clear evidence that electrons are accelerated to relativistic energies in supernova remnants (SNRs). However, a viable mechanism for the pre-acceleration of such electrons to mildly relativistic energies has not yet been established. In this paper an electromagnetic particle-in-cell (PIC) code is used to simulate acceleration of electrons from background energies to tens of keV at perpendicular collisionless shocks associated with SNRs. Free energy for electron energization is provided by ions reflected from the shock front, with speeds greater than the upstream electron thermal speed. The PIC simulation results contain several new features, including: the acceleration, rather than heating, of electrons via the Buneman instability; the acceleration of electrons to speeds exceeding those of the shock-reflected ions producing the instability; and strong acceleration of electrons perpendicular to the magnetic field. Electron energization takes place through a variety of resonant and non-resonant processes, of which the strongest involves stochastic wave-particle interactions. In SNRs the diffusive shock process could then supply the final step required for the production of fully relativistic electrons. The mechanisms identified in this paper thus provide a possible solution to the electron pre-acceleration problem.